I've been changing mine every other day. I only put about 1/2 cup full in each feeder and there isn't much left when I change it but it works better for me this way.

In this extreme heat no less than every other day, but some of my feeders are changed every day. This extreme heat will sour the mix very quickly. I have 8 feeders in my front yard and 1 30oz feeder in back which is shaded up till 2pm. I do not fill these feeders full on changing them so often , you have to know your hummers and about how much they will consume depending of course on how many you have.

My feeders hold 1 cup and they are emptied in a couple of days. They are in mostly shady locations, so I haven't had any problems.

I used to use larger feeders and leave them out longer, but I am pleased to say I have gotten out of that bad habit with help from the good advice I have received here. :)